This invention relates to a method of preparing 4-nitroisoxazoles. In particular, this invention relates to a novel method of preparing nitroisoxazoles in a single step by reacting a nitroketone and a hydroximyl halide.
Nitroisoxazoles can be prepared by initially forming a 1,2-oxazole and thereafter nitrating the 4-position. One method for preparing 1,2-oxazoles is by cyclization of monooximes of betadiketones with the simultaneous elimination of water. Nitration of the 4-position involves reaction of the 1,2-oxazoles with nitric acid suitably in the presence of sulfuric acid at elevated temperatures. While such a two-step method provides some yield of the desired 4-nitroisoxazole, the starting materials are usually highly reactive and frequently form a mixture of products that are not readily separated. Further, the nitration step is in many instances nonselective in nitrating the 4-position, as for example when the oxazole contains acyl or phenyl substituents. When nitration of a phenyl isoxazole is conducted, a mixture of aryl and isoxazole nitro-substituted products are formed. Nitration of, for example, 3-acyl-5-alkylisoxazoles oxidizes the acyl group to a carboxylic acid group. It appears, therefore, that such a method is applicable to the preparation of a limited number of nitroisoxazoles. Consequently, the method is not particularly economical or commercially attractive inasmuch as, in general, low yields of desired product are provided.
It is therefore an object of this invention to provide a novel method for preparing 4-nitroisoxazoles.
Another object of this invention is to provide a method for preparing 4-nitroisoxazoles in good yields.
Yet another object of this invention is to provide a method for preparing a wide range of 4-nitroisoxazoles in a single step.
Other objects and advantages will become apparent from the following detailed description and examples.